Driving force transmitting device for four-wheel drive vehicle

ABSTRACT

A driving force transmission device is for a four-wheel-drive vehicle based on the two-wheel drive of rear wheels or front wheels. In the case of the two-wheel drive of rear wheels, a multi-disc clutch mechanism controls the driving force distribution to a front wheel output shaft, and a disconnection/connection mechanism selectively disconnects and connects a front wheel differential and a left front wheel drive shaft. In the two-wheel drive of the rear wheels, the dragging torque of the multi-disc clutch mechanism is made smaller than the friction torque of a front wheel driving force transmission section, and the front wheel differential and the left front wheel drive shaft are disconnected by the disconnection/connection mechanism, thereby stopping the rotation of the front wheel driving force transmission section.

This application claims priority to prior application No. JP 2008-42409, filed Feb. 25, 2008 in Japan.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a driving force transmission device for a four-wheel-drive vehicle based on the two-wheel drive of rear wheels enabling change between two-wheel drive and four-wheel drive, and more specifically, to a driving force transmission device for a four-wheel-drive vehicle in which the rotation of the parts not operative for the transmission of driving force in the two-wheel drive mode is stopped.

2. Description of the Related Arts

As one of driving force transmission devices for four-wheel-drive vehicles in which rear wheels are driven in the two-wheel drive mode, and the distribution control of the driving force to the front wheels is performed in the four-wheel drive mode by a multi-disc clutch mechanism, in the four-wheel-drive vehicle based on the two-wheel drive of conventional so-called on-demand type full-time rear wheels, for example, the device shown in FIG. 1 is known.

In FIG. 1, a driving force transmission device 300 is provided in a four-wheel-drive vehicle 302, and the driving force from an engine 304 are changed by a gearbox 306 and input into a driving force distribution device 308 in the driving force transmission device 300. When a multi-disc clutch mechanism 310 is released (disconnected) in the two-wheel drive mode, the driving force is transmitted to a rear wheel differential 318 through a rear wheel propeller shaft 314 as it is, and the rear wheel differential 318 absorbs the rotation speed difference between a left rear wheel 320 and a right rear wheel 322, gives a same torque to the left rear wheel 320 and the right rear wheel 322, and makes the rear wheels rotate. When the multi-disc clutch mechanism 310 in fastened (connected) in the four-wheel drive mode, the driving force is transmitted also to a front wheel differential 324 through a chain belt mechanism 312 connected to the multi-disc clutch mechanism 310 and a front wheel propeller shaft 316, and the front wheel differential 324 absorbs the rotation speed difference between a left front wheel 326 and a right front wheel 328, gives a same torque to the left front wheel 326 and the right front wheel 328, and makes the front wheels rotate. In general, in the on-demand type full-time four-wheel-drive vehicle, as the drive modes for their drivers to optionally select by switch operations, a two-wheel-drive mode, a four-wheel-drive auto mode, and a four-wheel-drive lock mode are available. The two-wheel-drive mode is the mode that is used in the two-wheel-drive state in which the multi-disc clutch mechanism 310 of the driving force transmission device 300 is released, and selected when running on dry pavement roads on which the driving force by four wheels is not necessary for the best mileage. The four-wheel-drive auto mode is the mode in which various running vehicle states are detected by sensors, and the driving force distribution of the multi-disc clutch mechanism 310 to the front and rear wheels are automatically controlled at the most suitable state by an ECU (Electronic Control Unit) based on the detection signals of the sensor, and can be always selected regardless of road surface states. In this mode, the fastening power of the multi-disc clutch mechanism 310 is increased and/or decreased continually by an actuator, and the driving force distribution to the front and rear wheels is controlled between the two-wheel-drive state in which the driving force to the front wheels is approximately zero and the maximum fastening power. The four-wheel-drive lock mode is the mode in which the fastening power of the multi-disc clutch mechanism 310 is retained at its maximum regardless of the vehicle states detected by the various sensors, and this mode is selected when to show running characteristics as the four-wheel drive on rough roads and the like at the maximum. Meanwhile, in the present application, when it is not necessary to distinguish the modes definitely, the four-wheel-drive auto mode and the four-wheel-drive lock mode are collectively called four-wheel-drive modes. However, in such a conventional driving force transmission device for four-wheel-drive vehicle, since the left front wheel 326 and the right front wheel 328 and the front wheel differential 324 are directly connected, even in the two-wheel-drive mode, the left front wheel 326 and the right front wheel 328 still rotate in the two-wheel drive mode in which the multi-disc clutch mechanism 310 is released and the driving force is not transmitted to the front wheels as shown in FIG. 1. Consequently, the respective structural elements in the front wheel driving force transmission section 330 including the front wheel differential 324, the front wheel propeller shaft 316 and the chain belt mechanism 312 rotate too, and this leads to decreased mileage by stirring resistance of the oil in this section or friction loss of bearings, which has been a problem in the prior art. Further, the multi-disc clutch mechanism 310 includes a plurality of clutch discs, and although these clutch discs are lubricated and cooled by oil, since the so-called dragging torque that occurs by the viscosity resistance of the oil arising from the rotation speed difference between the driving side (rear-wheel side) and the driven side (front-wheel side) of the clutch discs and the friction loss by the contact between the clutch discs is larger than the friction torque of the front wheel driving force transmission section 330, even if the left front wheel 326 and the right front wheel 328 and the front wheel differential 324 are disconnected, the elements in the front wheel driving force transmission section 330 are rotated by the multi-disc clutch mechanism 310, and this deteriorates the mileage, which has been another problem in the prior art. In order to decrease this dragging torque, if the supply of the oil to the multi-disc clutch mechanism 310 is stopped, or if the multi-disc clutch mechanism is used with an extremely reduced quantity of the oil, it is possible to decrease or eliminate the dragging torque that occurs by the viscosity resistance of the oil. However, there is a fear that the multi-disc clutch mechanism 310 may seize if sufficient lubrication is not provided at the time of the driving force distribution control. Furthermore, as a method to ensure sufficient intervals among the respective clutch discs in the released state of the multi-disc clutch mechanism 310, a method is suggested in which a ring spring 332 formed in a wave shape in the circumferential direction as shown in FIG. 2 is put between adjacent clutch discs of the driving side, or adjacent clutch discs of the driven side. However, according to this method, it is possible to ensure the sufficient intervals between the clutch discs of the driving side or the clutch discs of the driven side, but it is not possible to prevent the clutch discs of the driving side from contacting the clutch discs of the driven side. In the four-wheel-drive auto mode, the driving force is transmitted to the front wheels 326 and 328, and even if the transmitted driving force is at its lowest, a loss larger than that at the time of the two-wheel-drive mode occurs in the front wheel driving force transmission section 330. Consequently, the mileage becomes worse than that at the time of the two-wheel-drive mode. In this mode, the driving force distribution is automatically controlled at the most suitable state even if the driver does not perform any operation. However, in the driving condition that does not need the four-wheel drive mode, such as running on dry pavement roads, when the driver forgets the switch change to the two-wheel-drive mode, the running is continued in a poor mileage state, which has been yet another problem in the prior art. In this case, by switching between the two-wheel-drive mode and the four-wheel-drive auto mode automatically depending on driving conditions, it is possible to improve the mileage regardless of the switch operation by the driver, but for that purpose, it is necessary to quickly change between the two-wheel drive mode and the four-wheel drive mode. As another method to decrease the dragging torque of the multi-disc clutch mechanism 310, there may be a method in which the distance of the approach section shifting from the standby state at the time of the two-wheel drive mode to the fastening state at the time of the four-wheel drive mode of the multi-disc clutch mechanism 310, that is, the so-called end play, is made large, and the intervals among the respective clutch discs are ensured sufficiently in the state in which the multi-disc clutch mechanism 310 is released at the time of the two-wheel drive mode, thereby the clutch discs are prevented from contacting one another. However, when the end play is made large, it takes a lot of time in changing the mode from the two-wheel drive mode to the four-wheel drive mode, which decreases the running performance.

SUMMARY OF THE INVENTION

According to the present invention, a driving force transmission device is provided for a four-wheel-drive vehicle which is based on the two-wheel drive of rear wheels, and does not sacrifice the change response from the two-wheel drive to the four-wheel drive, and decreases the oil viscosity resistance and friction loss owing to the rotation of the front wheel differential and the driving force distribution device in the two-wheel drive mode, thereby does not deteriorate the mileage.

According to an aspect of the present invention, there is provided a driving force transmission device for a four-wheel-drive vehicle based on a two-wheel drive of rear wheels, including

a switch mechanism for switching between a four-wheel-drive mode in which the fastening power of a multi-disc clutch mechanism is changed continually and the driving force distribution transmitted to front and rear wheels is controlled automatically depending on running conditions, and a two-wheel-drive mode in which the multi-disc clutch mechanism is released and the driving force is transmitted to only the rear wheels, and

a disconnection/connection mechanism for disconnecting and connecting a front wheel differential to which the driving force distributed by the multi-disc clutch mechanism is input and one or both of left and right front wheel drive shafts, wherein

the dragging torque of the multi-disc clutch mechanism is made smaller than the rotation resistance of a front wheel driving force transmission section from the multi-disc clutch mechanism to the disconnection/connection mechanism in the two-wheel-drive mode, and the front wheel differential and one or both of the left and right front wheel drive shafts are disconnected by the disconnection/connection mechanism and thereby the rotation of the front wheel driving force transmission section is stopped.

Herein, the multi-disc clutch mechanism includes a plurality of clutch discs that can be displaced in the axial direction of the multi-disc clutch mechanism, and a lubricating oil supply amount change mechanism to control the supply amount of lubricating oil to the clutch discs, wherein the supply of the lubricating oil to the clutch discs is stopped or limited to a specified amount by the lubricating oil supply amount change mechanism at the time when the multi-disc clutch mechanism is released, and the total of the intervals among the clutch discs is made above a specified value to make the dragging torque by the lubricating oil of the multi-disc clutch mechanism smaller than the rotation resistance of the front wheel driving force transmission section.

Further, the multi-disc clutch mechanism includes spacers urged in the direction to widen the intervals among the clutch discs, and the spacer is a plate spring member having a sandwiching unit to be inserted into a concave unit in the disc thickness direction provided in the circumferential part of the clutch disc, and an elastic piece that expands from the sandwiching unit to the radial direction of both the surfaces of the clutch disc and is deformable in the axial direction of the clutch disc. Furthermore, the plate spring member is provided in the internal circumferential part of the clutch disc, and in the plate spring member, the elastic piece is opened in the external circumferential direction of the clutch disc.

The lubricating oil supply amount change mechanism includes an oil pump that is driven in conjunction with the output to the rear wheels and supplies lubricating oil to the clutch discs, a press member that moves in the axial direction to fasten and release the clutch discs, and an oil hole opening/closing unit that opens and closes an oil hole to supply the lubricating oil to the clutch discs in conjunction with the press member and controls the flow amount of the lubricating oil, and the oil hole opening/closing unit closes the oil hole at the position where the clutch discs are released, and opens the oil hole at the position where the clutch discs are fastened.

The oil hole opening/closing unit includes an oil hole position adjustment member that changes the positions of the oil hole to supply the lubricating oil to the clutch discs in conjunction with the movement limit in the fastening direction of the press member which changes depending on the abrasion of the clutch discs.

Further, the multi-disc clutch mechanism includes a press member that moves in the axial direction to fasten and release the clutch discs, a drive source that outputs a rotation power to move the press member, and a press mechanism that converts and amplifies the rotation power from the drive source to the fastening direction of the press member, and the conversion rate to the axial direction displacement of the press member to the rotation angle of the drive source is higher in a shift section where the press member moves from the standby position in the releasing direction to the fastening start position in the fastening direction than in a press section where the fastening power is changed continually from the fastening start position to the fastening end position in the fastening direction.

The press member includes a ball cam mechanism in which a plurality of ball cam grooves expanding in the circumferential direction on facing surfaces of a fixed cam plate and a rotation cam plate provided coaxially with the press mechanism are provided and balls are sandwiched between the ball cam grooves, and the ball cam grooves have a nonlinear area to move the press member in the shift section, and a linear area to move the press member in the press section, and the displacement in the axial direction to the rotation angle of the rotation cam plate is larger in the nonlinear area than in the linear area.

The ball cam mechanism includes a retainer to retain the relative positions of the respective balls provided in the circumferential direction, and the retainer includes a positioning unit to regulate the initial positions of the balls to the ball cam grooves.

Further, the press mechanism includes a rotation cam plate provided coaxially with the press member to convert a rotation displacement into an axial direction displacement, a fringe cam having a cam surface in the external circumferential direction and driven to rotate by the drive source, and a cam follower to be engaged with the fringe cam at the end of an arm unit expanding from the rotation cam plate, and the cam surface includes a nonlinear area to move the press member in the shift section, and a linear area to move the press member in the press section, and the normal direction displacement of the cam follower to the rotation angle of the fringe cam is larger in the nonlinear area than in the linear area.

According to the present invention, in two-wheel-drive mode, the front wheel differential and one or both of the left and right front wheel drive shafts are disconnected by the disconnection/connection mechanism and the rotation of the ring gear of the front wheel differential accompanying the rotation of the front wheels is restrained, and the intervals among the clutch discs of the multi-disc clutch mechanism are ensured by the spacers, the supply of the oil to the clutch discs is limited, and the dragging torque of the multi-disc clutch mechanism is made smaller than the friction torque of the front wheel driving force transmission section. Thereby, it is possible to stop the rotation of the structural elements of the front wheel driving force transmission section, decrease the oil viscosity resistance and the friction loss in this section, and prevent the mileage from decreasing.

Furthermore, even when the end play is made large to ensure the interval of the clutch discs, the press mechanism is provided to move the press member faster in the approach section where the press member contacts or comes close to the clutch disc than in the press section thereafter, thereby, it is possible to prevent the mileage from decreasing, while not sacrificing the change response from two-wheel drive to four-wheel drive.

The above and other objects, features, and advantages of the present invention will become more apparent from the following detailed description with reference to the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory diagram showing a conventional driving force transmission device for four-wheel-drive vehicle based on the two-wheel drive of rear wheels;

FIG. 2 is an explanatory diagram showing a dragging prevention ring spring of a conventional multi-disc clutch mechanism;

FIG. 3 is an explanatory diagram showing an embodiment of a driving force transmission device for a four-wheel-drive vehicle based on the two-wheel drive of rear wheels according to the present invention;

FIGS. 4A and 4B are cross sectional views showing an embodiment of a front wheel differential in FIG. 1;

FIG. 5 is a cross sectional view showing an embodiment of a driving force distribution device in FIG. 3;

FIG. 6 is a cross sectional view showing a multi-disc clutch mechanism and a ball cam mechanism in FIG. 5;

FIG. 7 is an explanatory diagram showing the ball cam mechanism in FIG. 6 viewed from the axial direction;

FIGS. 8A to 8C are explanatory diagrams showing the movement of an oil hole opening/closing unit of the multi-disc clutch mechanism in FIG. 5;

FIGS. 9A and 9B are explanatory diagrams showing the movement of an oil hole position adjustment member of the oil hole opening/closing unit in FIG. 8;

FIG. 10 is an explanatory diagram showing a spacer setting state of the multi-disc clutch mechanism in FIG. 5;

FIG. 11 is a perspective view showing the spacer in FIG. 10;

FIGS. 12A and 12B are cross sectional views showing the spacer in FIG. 10;

FIGS. 13A and 13B are cross sectional views showing another embodiment of the spacer of the multi-disc clutch mechanism in FIG. 5;

FIGS. 14A to 14C are explanatory diagrams showing ball cam grooves and a retainer of the ball cam mechanism in FIG. 5;

FIG. 15 is a cross sectional view showing another embodiments of the driving force distribution device in FIG. 3;

FIGS. 16A and 16B are explanatory diagrams showing the ball cam mechanism in FIG. 15 viewed from the axial direction;

FIG. 17 is an explanatory diagram showing the movement of the ball cam mechanism shown in FIG. 14 and FIG. 16;

FIGS. 18A to 18C are explanatory diagrams showing the movement of the ball cam mechanism shown in FIG. 14 and FIGS. 16A and 16B;

FIG. 19 is an explanatory diagram showing another embodiment of a driving force transmission device for four-wheel-drive vehicle according to the present invention;

FIG. 20 is a cross sectional view showing an embodiment of the driving force distribution device in FIG. 19;

FIG. 21 is a cross sectional view showing another embodiment of the driving force distribution device in FIG. 19.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 3 is an explanatory diagram showing an embodiment of a driving force transmission device for a four-wheel-drive vehicle based on the two-wheel drive of rear wheels according to the present invention, and shows a case where the device is applied to a vehicle in which rear wheels are driven in the two-wheel-drive mode. In FIG. 3, a driving force transmission device 10 according to the present embodiment is provided in a four-wheel drive vehicle 12, and includes a driving force distribution device 18, a rear wheel differential 20 and a front wheel differential 22. The rear wheel differential 20 and the front wheel differential 22 are connected with the driving force distribution device 18 through a rear wheel propeller shaft 24 and a front wheel propeller shaft 26 respectively. The driving force from an engine 14 is changed by a gearbox 16, and input into an input shaft 102 of the driving force distribution device 18, and in the two-wheel-drive mode, when a multi-disc clutch mechanism 106 provided coaxially with the input shaft 102 is released, the driving force is output to a rear wheel output shaft 104 as it is, and transmitted to a drive pinion 36 of the rear wheel differential 20 through a universal joint 28, a rear wheel propeller shaft 24, and a universal joint 30. The drive pinion 36 drives a left rear wheel drive shaft 48 and a right rear wheel drive shaft 50 through a ring gear 38, pinions 40 and 42, side gears 44 and 46, and the left rear wheel drive shaft 48 and the right rear wheel drive shaft 50 rotate the a left rear wheel 52 and a right rear wheel 54 respectively and transmit the driving force to the road surface. Even if a rotation speed difference occurs between the left rear wheel 52 and the right rear wheel 54 owing to cornering or changes of the road surface states, the rear wheel differential 20 absorbs the rotation speed difference, gives a same torque to the left rear wheel 52 and the right rear wheel 54, and makes the rear wheels rotate. When the driver changes the mode from the two-wheel-drive mode to the four-wheel-drive auto mode, an ECU first fastens the multi-disc clutch mechanism 106, and then connects a disconnection/connection mechanism 76, thereby the driving force from the multi-disc clutch mechanism 106 is transmitted to a left front wheel 72 and a right front wheel 74. When the multi-disc clutch mechanism 106 is fastened, a sprocket 112 connected coaxially with the multi-disc clutch mechanism 106 rotates a sprocket 114 through a chain belt 116, and the driving force is transmitted also to the front wheel output shaft 118. The driving force output from the front wheel output shaft 118 is transmitted to a drive pinion 56 of the front wheel differential 22 through a universal joint 32, the front wheel propeller shaft 26, and a universal joint 34, and the drive pinion 56 drives a left front wheel drive shaft 68 and a right front wheel drive shaft 70 through a ring gear 58, pinions 60 and 62, side gears 64 and 66, and the left front wheel drive shaft 68 and the right front wheel drive shaft 70 rotate the left front wheel 72 and the right front wheel 74 respectively and transmits the driving force to the road surface. The disconnection/connection mechanism 76 connects the side gear 64 and the left front wheel drive shaft 68 in the four-wheel drive, and the rotation of the side gear 64 is transmitted to the left front wheel drive shaft 68 as it is. Even if a rotation speed difference occurs between the left front wheel 72 and the right front wheel 74 owing to cornering or changes of the road surface states, the front wheel differential 22 absorbs the rotation speed difference, gives a same torque to the left front wheel 72 and the right front wheel 74, and makes the front wheels rotate. The fastening power of the multi-disc clutch mechanism 106 is controlled continually by a servo motor, and the driving force distribution of the front and rear wheels is controlled by increasing and decreasing the driving force to be transmitted to the front wheel output shaft 118 according to need. When the mode is changed from the four-wheel-drive auto mode to the two-wheel-drive mode, the ECU first releases the multi-disc clutch mechanism 106, and then disconnects the disconnection/connection mechanism 76. In this case, the ECU may first disconnect the disconnection/connection mechanism 76, and then release the multi-disc clutch mechanism 106. The disconnection/connection mechanism 76 disconnects the side gear 64 and the left front wheel drive shaft 68, and prevents the rotation power that the left front wheel 72 and the right front wheel 74 receive from the road surface from rotating the ring gear 58. Thereby, it is possible to solve the problem that the front wheel driving force transmission section 78 from the ring gear 58 to the sprocket 112 rotates even in the two-wheel drive mode in which the front wheels are not driven, which is a factor in causing mileage decrease in the two-wheel drive mode. Further, even if the driver does not operate the changeover switch, by the judgment of the ECU to detect the vehicle state, the mode may be changed to the two-wheel-drive mode automatically when four-wheel drive is unnecessary, and on the contrary, the mode may be changed to the four-wheel-drive mode when four-wheel drive is necessary.

In FIG. 3, it is supposed that the side gear and the left front wheel drive shaft 68 are connected in the two-wheel-drive mode, for example, when the side gears 64 and 66 rotate in the same direction and at the same speed, the pinion 60 and the pinion 62 do not rotate (rotate on their axes), but the ring gear 58 rotates. Even if there is a rotation speed difference between the side gears 64 and 66, the rotation speed changes if they rotate in the same direction, but the ring gear 58 rotates, and consequently, the drive pinion 56, the universal joint 34, the front wheel propeller shaft 26, the universal joint 32, the front wheel output shaft 118, the sprocket 114, the chain belt 116, and the sprocket 112 connected thereto rotate. Although this front wheel driving force transmission section 78 from the ring gear 58 to the sprocket 112 is the part not required to rotate in two-wheel drive, the rotation of this part causes viscosity resistance of the oil and friction loss of bearings and the like. In other words, the driving force that is transmitted from the left rear wheel 52 and the right rear wheel 54 to the road surface rotates the left front wheel 72 and the right front wheel 74, and rotates the front wheel driving force transmission section 78 not required to rotate in two-wheel drive, which causes the loss of the driving force and invites the mileage decrease. Therefore, in the present invention, the connection between the side gear and the left front wheel drive shaft 68 is disconnected by the disconnection/connection mechanism 76 in the two-wheel-drive mode, and the dragging torque of the multi-disc clutch mechanism 106 is made smaller than the friction torque of the front wheel driving force transmission section 78, thereby the front wheel driving force transmission section 78 is prevented from rotating. When the connection between the side gear 64 and the left front wheel drive shaft 68 is disconnected, the rotation of the left front wheel 72 is not transmitted to the gear 64. Therefore, the rotation of the side gear 66 by the right front wheel 74 can rotate the side gear 64 through the pinion 60 and the pinion 62 in the opposite direction, and since the rotation resistance from the drive pinion 56 connected to the ring gear 58 to the sprocket 112 is larger than the rotation resistance of the pinion 60, the pinion 62, and the side gear 64, the ring gear 58 does not rotate. The fact that the ring gear 58 does not rotate means that the front wheel driving force transmission section 78 does not rotate, and the loss of the driving force in this case is limited in the parts where the pinion 60, the pinion 62, and the side gear 64 rotate. Accordingly, it is possible to improve the mileage more than in the case where there is not the disconnection/connection mechanism 76, and the front wheel driving force transmission section 78 rotates. Further, in the present embodiment, the disconnection/connection mechanism 76 is provided between the side gear 64 in the front wheel differential 22 and the left front wheel drive shaft 68, but the disconnection/connection mechanism may be provided in either the inside or the outside of the front wheel differential 22 as long as it is provided in the position to disconnect/connect the side gear 64 and the left front wheel drive shaft 68, or the position to disconnect/connect the side gear 66 and the right front wheel drive shaft 70, or in both of the positions. Furthermore, the mechanism may be another mechanism such as a method in which the unit to be constituted by the pinions 60 and 62, and the side gears 64 and 66 are separated from the ring gear 58, and the connection thereof is disconnected and connected.

FIGS. 4A and 4B are cross sectional views showing an embodiment of the front wheel differential 22 in FIG. 4, where the disconnection/connection mechanism 76 is provided. In FIGS. 4A and 4B, the front wheel differential 22 includes the ring gear 58 fixed to the external circumferential part of a differential gear case 80, a pinion 60 and a pinion 62 supported rotatably by a pinion shaft 82 fixed to the differential gear case 80, a side gear 64 supported unrotatably by a side gear shaft 84 and engaged with the pinion 60 and the pinion 62 in the differential gear case 80, and a side gear 66 supported unrotatably by the right front wheel drive shaft 70 and engaged with the pinion 60 and the pinion 62 in the differential gear case 80. Furthermore, the front wheel differential 22 includes a left front wheel drive shaft 68 whose end 68 b engaged with the side gear shaft 84 while not restricted by rotation directions, a sleeve 86 splined to a tooth unit 68 a of the left front wheel drive shaft 68 and a tooth unit 84 a of side gear shaft 84, and slidable between the position to connect the left front wheel drive shaft 68 and the side gear shaft 84 and the position to disconnect them, a fork 88 for sliding the sleeve 86 by an end 88 a that is slidably engaged with a groove unit 86 a of the sleeve 86, and a shift shaft fixed to the fork 88 and driven in the axial direction by an actuator not shown, and transmits the driving force from the drive pinion 56 engaged with the ring gear 58 in four-wheel drive to the left front wheel 72 and the right front wheel 74.

In FIG. 4A, the disconnection/connection mechanism 76 in the two-wheel drive is in non-connection state, and the sleeve 86 is not engaged with tooth unit 84 a of the side gear shaft 84. The rotation of the right front wheel drive shaft 70 is transmitted to the side gear 64 through the side gear 66, the pinion 60 and the pinion 62, and because the ring gear 58 does not rotate, the side gear shaft 84 is rotated in the direction opposite to the rotation direction of the right front wheel drive shaft 70.

In FIG. 4B, in the state where the fork 88 moves to the L direction in the four-wheel drive, and the disconnection/connection mechanism 76 is connected thereto, the sleeve 86 is engaged with the tooth unit 84 a of the side gear shaft 84. The ring gear 58 rotates by the drive pinion 56, and rotates the left front wheel drive shaft 68 and the right front wheel drive shaft 70 in the same direction. When the mode is changed back to the two-wheel drive, the fork 88 moves to the U direction, and the disconnection/connection mechanism 76 is changed back to the non-connection state. As another factor to cause the mileage decrease in the two-wheel-drive mode, there is the problem that the front wheel driving force transmission section 78 rotates because the so-called dragging torque that occurs by the viscosity resistance of the oil arising from the rotation speed difference between the driving side (rear-wheel side) and the driven side (front-wheel side) of the clutch discs provided in the multi-disc clutch mechanism and the friction loss by the contact between the clutch discs is larger than the friction torque of the front wheel driving force transmission section 78.

FIG. 5 is a cross sectional view showing an embodiment of the driving force distribution device 18 in FIG. 3. In FIG. 5, the driving force distribution device 18 includes a case 100, and an input shaft 102 that inputs the driving force from the engine 14 into the left side of the case 100 through the gearbox 16 is provided, and the input shaft 102 is connected directly with a rear wheel output shaft 104 provided in the right side of the case 100. A multi-disc clutch mechanism 106 and a ball cam mechanism 122 are provided coaxially with the input shaft 102, and the multi-disc clutch mechanism 106 fixes a clutch hub 108 to the input shaft 102, and connects a clutch drum 110 to a sprocket 112 provided rotatably to the input shaft 102. In parallel with the input shaft 102, a front wheel output shaft 118 for outputting the driving force to the opposite side of the rear wheel output shaft 104 is provided in the left bottom of the case 100, a sprocket 114 is connected to the front wheel output shaft 118, and a chain belt 116 is provided between the sprocket 114 and a sprocket 112 at the side of the multi-disc clutch mechanism 106. An oil pump 184 is provided at the side of the rear wheel output shaft 104 of the ball cam mechanism 122, and gets power from the input shaft 102 and supplies oil to an oil path 186 provided in the input shaft 102. In such a driving force distribution device 18, the clutch hub 108 and the clutch drum 110 of the multi-disc clutch mechanism 106 are released in the two-wheel drive mode, and the driving force of the input shaft 102 is transmitted directly to the rear wheel output shaft 104. In the four-wheel drive mode, the multi-disc clutch mechanism 106 is fastened, and the driving force from the input shaft 102 is transmitted also to the front wheel output shaft 118 through the multi-disc clutch mechanism 106, the sprocket 112, the chain belt 116, and the sprocket 114.

FIG. 6 is a cross sectional view showing the multi-disc clutch mechanism 106 and the ball cam mechanism 122 in FIG. 5. In FIG. 6, to the multi-disc clutch mechanism 106, a ball cam mechanism 122 to control the fastening power of the clutch discs 120 provided between the clutch hub 108 and the clutch drum 110 is provided. The clutch discs 120 includes spacers 178 for retaining intervals among the clutch discs. In the ball cam mechanism 122, a ball 140 is sandwiched in ball cam grooves 128 of the cam surfaces of a pair of a fixed cam plate 124 and a rotation cam plate 126 provided relatively rotatably and coaxially with the input shaft 102. Furthermore, a retainer 136 having a projection 138 for positioning is provided between the fixed cam plate 124 and the rotation cam plate 126.

FIG. 7 is an explanatory diagram showing the ball cam mechanism 122 in FIG. 5 viewed from the direction of the rear wheel output shaft 104. In FIG. 7, in the fixed cam plate 124, a U-shaped end of its expanded arm unit 142 is engaged with the outside of a retaining unit 152 a, thereby its rotation is stopped. The rotation cam plate 126 includes a plurality of ball cam grooves 128 in its circumferential direction, and a fan type gear 146 formed on the end of its arm unit 144 is engaged with a drive gear 148.

With reference to FIGS. 5 and 6 again, the servomotor 150 rotates the drive gear 148 through a reduction gear 152. A fixed plate 158 is provided in the right side of fixed cam plate 124 through a thrust bearing 154, and a press member 160 is provided at the left side of the rotation cam plate 126 through a thrust bearing 156. The press member 160 is urged to the releasing direction of the multi-disc clutch mechanism 106 by a return spring 162 provided between the clutch hub 108 and the press member. When the rotation cam plate 126 is driven to rotate to the fixed cam plate 124 in the specified direction by the drive gear 148, the ball cam mechanism 122 receives the press by the ball 140 sandwiched between the ball cam grooves 128 which are the inclined grooves of the facing surfaces and presses the press member 160 and the return spring 162 in the axial direction, and the press member 160 presses the clutch discs 120 of the multi-disc clutch mechanism 106, and the ball cam mechanism increases the transmission torque depending on the rotation amount of the drive gear 148, therefore it enters direct connection state at the maximum press position. In the part surrounded by the input shaft 102, the clutch hub 108, and the press member 160, an oil hole opening/closing unit 164 for controlling the supply of the oil to the plurality of clutch discs 120 of the multi-disc clutch mechanism 106 from an oil path 186 is provided. The oil hole opening/closing unit 164 includes an oil hole 168, an oil hole position adjustment member 166 having a ratchet claw 170, a disc spring 172, a rack 174 provided in the input shaft 102, and a valve 176 provided in the press member 160 to open and close the oil hole 168.

FIGS. 8A to 8C are explanatory diagrams showing the movement of an oil hole opening/closing unit 164 of the multi-disc clutch mechanism 106 in FIG. 3. FIG. 8A shows a state where the multi-disc clutch mechanism 106 is released in the two-wheel drive mode, and the press member 160 is at standby position most away from the clutch discs 120. The oil hole position adjustment member 166 is urged to the right side by the disc spring 172 and regulated by the ratchet claw 170 and the rack 174. In this state, the clutch discs engaged with the clutch drum 110 among the clutch discs 120 do not rotate, but the clutch discs engaged with the clutch hub 108 rotate in conjunction with the rotation of the input shaft 102. However, when the press member 160 is at this position, the valve 176 closes the oil hole 168, and accordingly, the oil from the oil path 186 is not supplied to the clutch discs 120, and thereby, it is possible to prevent the dragging at the side of the clutch drum 110 by reducing the viscosity resistance of the oil. Further, in the present embodiment, the valve 176 closes the oil hole 168 completely in the state where the multi-disc clutch mechanism 106 in FIG. 8A is released, but at this moment, the valve 176 may not close the oil hole 168 completely, and some amount of oil may be supplied to the clutch discs 120, and the amount may be optionally set in consideration of the lubrication and the cooling of the clutch discs 120.

FIG. 8B shows the fastening initial state of the multi-disc clutch mechanism 106 on the way of the change from the two-wheel drive mode to the four-wheel drive mode, and the press member 160 which is pressed by the ball cam mechanism 122 and moved to the left side is at the position to start the contact to the clutch discs 120. From this point in time, the clutch discs 120 gradually start contacting one another, and by the friction torque thereof, the multi-disc clutch mechanism 106 starts transmitting the driving force to the front wheel side. Because the valve 176 opens the oil hole 168 at this position, the oil is supplied from the oil path 186 to the clutch discs 120 as shown by arrows, and thereby, the clutch discs are prevented from seizing.

FIG. 8C shows the state where the mode is changed into the four-wheel drive mode, and the multi-disc clutch mechanism 106 is completely fastened, and the press member 160 is at maximum press position where there is no interval between the clutch discs 120 at all. In the oil hole position adjustment member 166, a projection 166 a is pressed to the valve 176, and the ratchet claw 170 is engaged with and retained by the rack 174 at the maximum press position of the press member 160.

FIGS. 9A and 9B are explanatory diagrams showing the movement of the oil hole position adjustment member 166 of the oil hole opening/closing unit 164 in FIGS. 8A to 8C. FIG. 9A show the oil hole opening/closing unit 164 in the state where the clutch discs 120 are not abraded, or in the initial state of the abrasion, and FIG. 9B shows the oil hole opening/closing unit 164 in the state where the abrasion of the clutch discs 120 progresses. When the abrasion of the clutch discs 120 progress from FIG. 9A to FIG. 9B, the maximum press position of the press member 160 moves to the direction where the clutch stroke becomes deep by the distance roughly same as the total of the decreased disc thickness of all the clutch discs 120, that is, to the left side by a distance W in FIG. 6. In conjunction with the movement of the maximum press position of the press member 160, the position where the projection 166 a of the oil hole position adjustment member 166 is pressed to the valve 176 changes to the left side, and at the position, the ratchet claw 170 is engaged with the rack 174, and the oil hole position adjustment member 166 is retained. Since the oil hole position adjustment member 166 follows the abrasion of the clutch disc 120, the timing of the opening start of the oil hole 168 is not influenced by the abrasion degree of the clutch disc 120, and becomes roughly constant to the intervals among the clutch disc 120, and accordingly it is possible to stabilize the oil supply amount that gives influences to the lubrication, cooling, dragging and the like of the clutch disc 120.

FIG. 10 is an explanatory diagram showing the setting state of the spacers 178 of the multi-disc clutch mechanism 106 in FIG. 5. FIG. 10 shows the state where the multi-disc clutch mechanism 106 is released in the two-wheel drive mode, and the press member 160 is at the standby position furthest away from the clutch disc 120. The multi-disc clutch mechanism 106 includes a plurality of clutch discs 120 which can move in the axial direction, and in the clutch discs 120, inside clutch discs 120 h are splined to the external circumference of the clutch hub 108, and outside clutch discs 120 d are splined to the internal circumference of the clutch drum 110 and the inside clutch discs 120 h and the outside clutch discs 120 d are provided alternately. Spacers 178 which are plate spring molded of a plate material are provided in the internal circumferential parts of the outside clutch discs 120 d, and ensure the intervals among the frictional surfaces not to cause the dragging with the clutch discs 120 h at both the adjacent sides. The clutch discs 120 can move in the axial direction between a pressed surface 108 a of the clutch hub 108 and the pressing surface 160 a of the press member 160, and there is an interval E to the press side 160 a. Meanwhile, without the spacers 178, the clutch discs 120 adhere one another by the oil and do not separate, and cause the dragging. Since the spacers 178 ensure the intervals among the frictional surfaces, it is possible to prevent the loss of the driving force owing to the dragging and improve the mileage. Further, the value of the interval E at the moment when the member 160 is at the standby position farthest away from the clutch discs 120 may not necessarily be constant by the states of the intervals among the clutch discs 120. The spacers 178 may be provided in the external circumferential parts of the inside clutch discs 120 h, but it is preferable to provide the same in the inside with a small torque loss. In other words, with a same frictional resistance by sliding between the clutch discs 120 adjacent to the clutch discs 120 in which the spacers 178 are provided, and the spacers 178, the friction torque is smaller when the spacers are provided in the inside, and the friction torque becomes the loss of the driving force. Further, it is preferable that the spring load of the spacers 178 is as small as in the range that the intervals to the adjacent clutch discs 120 h can be retained.

FIG. 11 is a perspective view showing the spacer in FIG. 10. In FIG. 11, the spacer 178 is a plate spring molded of a plate material, and includes a sandwiching unit 180 to be inserted into a concave unit 120 c provided in the plate direction in the internal circumferential part of the outside clutch disc 120 d, and an elastic piece 182 that expands from the sandwiching unit 180 in the outer diameter direction of both the surfaces of the adjacent clutch discs 120, and can be transformed in the axial direction of the clutch discs 120. The interval of the sandwiching unit 180 is set narrower than the thickness of concave unit 120 c in order to insert and fix the spacer 178 into the concave unit 120 c in the direction shown by an arrow.

FIGS. 12A and 12B are cross sectional views showing the spacer 178 of FIG. 10. FIG. 12A shows the state where the multi-disc clutch mechanism 106 is fastened in the four-wheel drive mode, and the respective frictional surfaces of the outside clutch disc 120 d in which the spacer 178 is provided and the inside clutch discs 120 h adjacent thereto are closely attached for the rotation. FIG. 13B shows the state where the multi-disc clutch mechanism 106 is released in the two-wheel drive, and the respective frictional surfaces of the outside clutch disc 120 d in which the spacer 178 is provided and the inside clutch discs 120 h adjacent thereto are separated apart to the intervals not to cause the dragging by the spacer 178.

In FIG. 12B, since the inside clutch discs 120 h rotate, and the outside clutch disc 120 d does not rotate, the ends of the elastic pieces 182 of the spacer 178 slide with the adjacent inside clutch discs 120 h. Therefore, it is preferable for the ends of the elastic piece 182 to come in contact with the part that is not a frictional surface.

FIGS. 13A and 13B are cross sectional views showing another embodiment of the spacer 178 of the multi-disc clutch mechanism 106 in FIG. 5. In FIGS. 13A and 13B, the spacer 178 is a plate spring molded of a plate material, and includes a sandwiching unit 180 to be inserted into a concave unit 120 c provided in the plate direction in the internal circumferential part of the outside clutch disc 120 d, and an elastic piece 182 that expands from the sandwiching unit 180 in the inner diameter direction of both the surfaces of the adjacent inside clutch discs 120 h, and can be transformed in the axial direction of the inside clutch discs 120 h.

FIG. 13A shows the state where the multi-disc clutch mechanism 106 is fastened, and the respective frictional surfaces of the outside clutch disc 120 d in which the spacer 178 is provided and the inside clutch discs 120 h adjacent thereto is closely attached. FIG. 13B shows the state where the multi-disc clutch mechanism 106 is released, and the respective frictional surfaces of the outside clutch disc 120 d in which the spacer 178 is provided and the inside clutch discs 120 h adjacent thereto are separated apart to the intervals not to cause the dragging by the spacer 178. In the middle position between FIG. 13A and FIG. 13B, the inside clutch discs 120 h and the outside clutch disc 120 d rotate together, but since there is a rotation speed difference, the ends of the elastic piece 182 slide with the adjacent inside clutch discs 120 h. However, the ends of the elastic piece 182 receives a force in the direction to close as shown by arrows, by a centrifugal force by the rotation of the outside clutch disc 120 d, and thereby, it is possible to reduce the frictional resistance.

FIGS. 14A to 14C are explanatory diagrams the showing ball cam grooves 128 and a retainer 136 of the ball cam mechanism 122 in FIG. 5. In FIGS. 14A to 14C, a ball 140 is sandwiched and retained in ball cam grooves 128 in facing cam surfaces of a pair of a fixed cam plate 124 and a rotation cam plate 126. The ball cam groove 128 includes an initial area 130 and an inclined area 132 whose cam shapes are different with a runover unit 134 as the border. Further, a retainer 136 for retaining the relative position of the ball 140 is provided between the fixed cam plate 124 and the rotation cam plate 126, and to the retainer 136, a projection 138 for positioning to regulate the initial position of the ball 140 to the ball cam groove 128 is provided. The projection 138 is engaged with a guide unit 127 formed on the external circumferential part of the rotation cam plate 126.

FIG. 14A shows the state where the multi-disc clutch mechanism 106 is released in the two-wheel drive mode and the press member 160 is at the standby position, and the ball 140 is positioned in the initial area 130 of both the ball cam grooves 128 of the fixed cam plate 124 and the rotation cam plate 126.

FIG. 14B shows the state where the press member 160 starts contacting the clutch disc 120 in the fastening initial state of the multi-disc clutch mechanism 106 when the mode shifts from the two-wheel drive mode to the four-wheel drive mode, and the ball 140 is at the position shifted from the runover unit 134 to the inclined part 132 of the ball cam grooves 128.

FIG. 14C shows the state where the mode is changed to the four-wheel drive mode, the multi-disc clutch mechanism 106 is fastened, and the press member 160 is at the position to press the clutch discs 120, and the ball 140 is positioned at the inclined parts 132 of the ball cam grooves 128. When a plurality of balls 140 are provided on the circumference in order to disperse the load, there may be a relative displacement among the balls owing to the slide of the ball cam grooves 128 and the balls 140 at the moment of the action of the ball cam mechanism 122, and in that case, it is not possible to press the ball cam grooves 128 equally, and a biased load occurs in the multi-disc clutch mechanism 106 and the surface durability of the ball cam grooves 128 is conspicuously deteriorated. By providing the retainer 136, the relative position of the respective balls 140 is retained, and at every moment the balls return from FIG. 14C to the initial position of FIG. 14A, the initial position of the respective balls 140 to the ball cam grooves 128 are corrected by the projection 138 for the positioning.

FIG. 15 is a cross sectional view showing another embodiment of the driving force distribution device 18 in FIG. 3, and the configuration is same as that shown in FIGS. 5 and 6 except that a ball cam mechanism 122 is different. In FIG. 16, a servo motor 150 rotates a fringe cam 198 through a reduction gear 152. In the rotation cam plate 126, a cam follower 196 consisting of a rotatable roller is provided on the end of an expanded arm unit 144, and the cam follower 196 contacts the external circumferential part of the fringe cam 198. When the rotation cam plate 126 is driven to rotate to the fixed cam plate 124 in the specified direction by the fringe cam 198, the ball cam mechanism 122 receives the press by the ball 140 sandwiched between the ball cam grooves 128 which are the inclined grooves of the facing surfaces and presses the press member 160 and a return spring 162 in the axial direction, and the press member 160 presses the clutch discs 120 of the multi-disc clutch mechanism 106, thereby the ball cam mechanism increases the transmission torque depending on the rotation amount of the drive gear 148, therefore it enters direct connection state at the maximum press position.

FIGS. 16A and 16B are explanatory diagrams showing the ball cam mechanism 122 in FIG. 15 viewed from the axial direction of the input shaft 102. In FIGS. 16A and 16B, the rotation cam plate 126 includes the ball cam grooves 128 in the circumferential direction. The ball cam grooves 128 in the present embodiment do not have a runover unit 134 as shown in FIGS. 14A to 14C in the embodiment of FIG. 3, and the inclined area 132 has a shape that goes without a step from the initial area 130. The external circumferential part of a cam follower 196 provided on the end of an arm unit 144 expanding from the rotation cam plate 126 contacts the cam surface of a fringe cam 198. The cam surface of the fringe cam 198 consists of an initial area 200, a nonlinear area 202 and a linear area 206, and the fringe cam 198 rotates clockwise to drive and rotate the rotation cam plate 126 through the cam follower 196, and presses the multi-disc clutch mechanism 106.

FIG. 16A shows the state where the multi-disc clutch mechanism 106 is released in the two-wheel drive mode, and the press member 160 is at the standby position, and the cam follower 196 is positioned in the initial area 200 of the cam surface of the fringe cam 198.

FIG. 16B shows the state where the press member 160 starts contacting the clutch disc 120 in the fastening initial state of the multi-disc clutch mechanism 160 when the mode shifts from the two-wheel drive mode to the four-wheel drive mode, and the cam follower 196 is at the position shifting from the nonlinear area 202 to the linear area 206 of the cam surface of the fringe cam 198, and this point becomes the press start position 204. The press start position 204 of the fringe cam 198 may be at the nonlinear area 202 or the linear area 206 as long as it is near the position shifting from the nonlinear area 202 to the linear area 206, but it is preferable that the press start position is at a point shifting from the nonlinear area 202 to the linear area 206. Herein, the shape of the linear cam 208 without the nonlinear area is shown as a prior-art example and compared with the fringe cam 198. In the linear cam 208, an angle β is required until the press member 160 reaches the press start position 210 to start pressing the clutch disc 120, meanwhile in the fringe cam 198 having the nonlinear area 202, the press member reaches the press start position 204 by an angle α that is far smaller than the angle β.

FIG. 17 and FIGS. 18A to 18C are explanatory diagrams showing the movement of the ball cam mechanism 122 shown in FIGS. 14A to 14C and FIGS. 16A and 16B. FIG. 17 is a graph schematically showing the relation between the cam load working to the press member 160 of the multi-disc clutch mechanism 106, and the stroke 190 of the press member 160, and the spring load 192, and the horizontal axis shows the cam angle (θ) of the rotation cam plate 126, and the vertical axis shows the cam load (F) and the stroke (S). The stroke 190 of the press member 160 shows a nonlinear area from the cam angle θ₀ to θ₁, and a linear area after the cam angle θ₁ to θ_(max), and the axis of the clutch torque (T) includes its start point (T0) at the spring load 192 of the cam angle θ₁, and is perpendicular to the inclination of the spring load 192.

FIG. 18A shows the states of the press member 160, the clutch drum 108, the clutch disc 120 and the return spring 162 corresponding to the cam angle θ shown in FIG. 18, and the states are the standby state θ₀, fastening start θ₁, and fastening state θ_(n) of the multi-disc clutch mechanism 106 respectively.

FIG. 18B shows the positional relations of the fixed cam plate 124, the rotation cam plate 126, and the ball 140 shown in FIGS. 14A to 14C corresponding to the cam angle θ, and the relations show, from the left to the right, the case where the ball 140 of the cam angle θ₀ is at the initial area 130 of the ball cam grooves 128, the case where the ball 140 of the cam angle θ₁ is at the position where the ball 140 of the cam angle θ₁ finishing running over the runover areas 134 of the ball cam grooves 128, and the state where the ball 140 of the cam angle θ, is in the inclined area 132 of the ball cam grooves 128 respectively.

FIG. 18C shows the positional relations of the fringe cam 188 and the cam follower 196 positioned at the end of the arm unit 144 of the rotation cam plate 126 shown in FIGS. 16A and 16B corresponding to cam angle θ, and the relations show, from the left to the right, the case where the cam follower 196 of the cam angle θ₀ is at the initial area 200 of the fringe cam 198, the case where the cam follower 196 of the cam angle θ₁ is at the press start position 204 of the fringe cam 198, and the case where the cam follower 196 of the cam angle θ_(n) is at the linear area 206 of the fringe cam 198 respectively. In the multi-disc clutch mechanism 106, although the movement of the press member 160 is made with a small load only as to press the return spring 162 from the standby position to the position to start contacting the clutch discs 120, a high speed is necessary in order to have good shift response from the two-wheel drive mode to the four-wheel drive mode. In particular, when the intervals among the clutch discs 120 are made large to prevent the dragging in the two-wheel drive mode, and the end play of the multi-disc clutch mechanism 106 is made large, the movement speed of this section becomes important. In the standby state of the cam angle θ₀, there is an end play of a distance e between the press member 160 and the clutch discs 120, and in the case of FIG. 18B, this distance e is quickly shortened in the nonlinear area where the ball 140 runs over the runover unit 134 of the ball cam grooves 128 from the cam angle θ₀ to the cam angle θ₁. In the case of FIG. 19C, the cam follower 196 quickly shortens this distance e by passing the nonlinear area 202 from the cam angle θ₀ to the cam angle θ₁ and moving to the press start position 204. Since the cam load (F) in the section from the cam angle θ₀ to the cam angle θ₁ is only the spring load 192 of the return spring 162, it is possible to obtain a large stroke S₁ with the small cam angle θ₁. Thereafter, when the press member 160 starts contacting and pressing the clutch disc 120, a clutch torque (T) occurs in the clutch discs 120, and the cam load (F) increases rapidly. After the cam angle θ₁, the ball 140 is in the inclined areas 132 of the ball cam grooves 128, and the value obtained by subtracting the spring load 192 from the load Fn, in the stroke Sn of the cam angle θ_(n), is equivalent to the clutch torque Tn.

FIG. 19 is an explanatory diagram showing another embodiment of a driving force transmission device for four-wheel-drive vehicle according to the present invention, and the configuration is same as that in the embodiment shown in FIG. 3 except for an oil pump 194. In FIG. 19, the oil pump 194 is connected to the sprocket 114, and when the multi-disc clutch mechanism 106 is fastened and the sprocket 114 rotates, the oil pump 194 supplies the oil to the multi-disc clutch mechanism 106 through a hydraulic pressure circuit not shown, and prevents seizing owing to the frictional heat of the plurality of the clutch discs provided in the multi-disc clutch mechanism 106. Meanwhile, in the present embodiment, the oil pump 194 is connected to the sprocket 114 and gets power therefrom, but it may get power from the portions to which the driving force is not transmitted in the two-wheel drive mode, for example, the sprocket 112 or the chain belt 116.

FIG. 20 is a cross sectional view showing an embodiment of the driving force distribution device 18 in FIG. 19, and the configuration is same as that of the embodiment shown in FIG. 5 except that there is not the oil pump 184 which the input shaft 102 drives, and instead, there is the oil pump 194 which the sprocket 114 drives, and there is not the oil hole opening/closing unit 164 whose details are shown in FIG. 6. In FIG. 20, the oil pump 194 is provided in the opposite side of the front wheel output shaft 118 of the sprocket 114, gets power from the shaft of the sprocket 114, and supplies the oil to the oil path 186 provided in the input shaft 102 through a hydraulic pressure circuit not shown. In such a configuration, when the multi-disc clutch mechanism 106 is released in the two-wheel drive mode, and the press member 160 is at standby position furthest away from the clutch discs 120, the clutch discs engaged with the clutch drum 110 among the clutch discs 120 do not rotate, but the clutch discs engaged with the clutch hub 108 rotate in conjunction with the rotation of the input shaft 102. However, because the clutch drum 110 does not rotate, and the sprocket 114 through the sprocket 112 and the chain belt 116 does not rotate, and the oil pump 194 does not discharge the oil to the oil path 186. Therefore, the oil from the oil path 186 is not supplied to the clutch discs 120, and the viscosity resistance by the oil is reduced, and thereby, it is possible to prevent the dragging at the side of the clutch drum 110. In the state where the mode is changed from the two-wheel drive mode to the four-wheel drive mode, the press member 160 which is pressed by the ball cam mechanism 122 and moves to the left side presses the clutch discs 120, and the clutch discs 120 contact one another, since the clutch drum 110 rotates, the sprocket 114 rotates through the sprocket 112 and the chain belt 116. Therefore, the oil pump 194 driven by the sprocket 114 discharges the oil to the oil path 186, and the oil is supplied from the oil path 186 to the clutch discs 120 to prevent the clutch discs from seizing one another.

FIG. 21 is a cross sectional view showing another embodiment of the driving force distribution device 17 in FIG. 19, and the configuration is same as that of the embodiment shown in FIG. 20 except that there is an oil pump 212 that a sprocket 112 drives in the place of the oil pump 194 driven by the sprocket 114. In FIG. 22, the oil pump 212 is provided in the side of the input shaft 102 of the sprocket 112, gets power from the shaft of the sprocket 112, and supplies oil to the oil path 186 provided in the input shaft 102. According to this configuration, it is possible to obtain the same effects as those by the configuration shown in FIG. 20.

The present invention is not limited to the embodiment mentioned above, but includes an arbitrary modification not impair the object and advantage thereof, and further the present invention is not limited to the values indicated in the above embodiments. 

1. A driving force transmission device for a four-wheel-drive vehicle, the driving force transmission device comprising: a switch mechanism for switching between a four-wheel-drive mode in which a fastening power of a multi-disc clutch mechanism is changed continually and a driving force distribution to front and rear wheels is automatically controlled depending on running conditions, and a two-wheel-drive mode in which the multi-disc clutch mechanism is released and the driving force distribution is transmitted only to the rear wheels; and a disconnection/connection mechanism for disconnecting and connecting a front wheel differential to which the driving force distributed by the multi-disc clutch mechanism is input and one or both of left and right front wheel drive shafts, wherein a dragging torque of the multi-disc clutch mechanism is made smaller than a rotation resistance of a front wheel driving force transmission section from the multi-disc clutch mechanism to the disconnection/connection mechanism in the two-wheel-drive mode, and the front wheel differential and the one or both of the left and right front wheel drive shafts are disconnected by the disconnection/connection mechanism and thereby a rotation of the front wheel driving force transmission section is stopped.
 2. The driving force transmission device for a four-wheel-drive vehicle according to claim 1, wherein the multi-disc clutch mechanism includes: a plurality of clutch discs that can be displaced in an axial direction of the multi-disc clutch mechanism; and a lubricating oil supply amount change mechanism that controls a supply amount of lubricating oil to the clutch discs, and a supply of the lubricating oil to the clutch discs is stopped or limited to a specified amount by the lubricating oil supply amount change mechanism at a time when the multi-disc clutch mechanism is released, and a distance of intervals among the clutch discs is above a specified value to make the dragging torque by the lubricating oil of the multi-disc clutch mechanism smaller than the rotation resistance of the front wheel driving force transmission section.
 3. The driving force transmission device for a four-wheel-drive vehicle according to claim 2, wherein the multi-disc clutch mechanism includes spacers that urge in a direction to widen the intervals among the clutch discs.
 4. The driving force transmission device for a four-wheel-drive vehicle according to claim 3, wherein each of the spacers is a plate spring member including: a sandwiching unit to be inserted into a concave unit in a disc thickness direction provided in an edge of a respective one of the clutch discs; and an elastic piece that expands from the sandwiching unit to a radial direction of both surfaces of the respective one of the clutch discs and is deformable in an axial direction of the clutch discs.
 5. The driving force transmission device for a four-wheel-drive vehicle according to claim 4, wherein each of the spacers is provided at an internal circumferential part of the respective one of the clutch discs.
 6. The driving force transmission device for a four-wheel-drive vehicle according to claim 4, wherein the elastic piece of each of the spacers is opened in an external circumferential direction of the respective one of the clutch discs.
 7. The driving force transmission device for a four-wheel-drive vehicle according to claim 2, wherein the lubricating oil supply amount change mechanism includes: an oil pump that is driven in conjunction with an output to the rear wheels and supplies the lubricating oil to the clutch discs; a press member that moves in the axial direction of the clutch disks to fasten and release the clutch discs; and an oil hole opening/closing unit that opens and closes an oil hole that supplies the lubricating oil to the clutch discs in conjunction with the press member to control the supply amount of the lubricating oil, and the oil hole opening/closing unit closes the oil hole at a position where the clutch discs are released, and opens the oil hole at a position where the clutch discs are fastened.
 8. The driving force transmission device for a four-wheel-drive vehicle according to claim 7, wherein the oil hole opening/closing unit includes an oil hole position adjustment member that changes positions of the oil hole that supplies the lubricating oil to the clutch discs in conjunction with a movement limit in a fastening direction of the press member which changes depending on an abrasion state of the clutch discs.
 9. The driving force transmission device for a four-wheel-drive vehicle according to claim 2, wherein the lubricating oil supply amount change mechanism includes an oil pump that is driven in conjunction with the front wheel driving force transmission section and supplies the lubricating oil to the clutch discs in the four-wheel-drive mode.
 10. The driving force transmission device for a four-wheel-drive vehicle according to claim 2, wherein the multi-disc clutch mechanism further includes: a press member that moves in an axial direction of the clutch discs to fasten and release the clutch discs; a drive source that outputs a rotation power to move the press member; and a press mechanism that converts and amplifies the rotation power from the drive source to a fastening direction of the press member, and a the conversion rate of an axial direction displacement of the press member to a rotation angle of the drive source is higher in a shift section where the press member moves from a standby position in a releasing direction to a fastening start position in the fastening direction than in a press section where fastening power is changed continually from the fastening start position to a fastening end position in the fastening direction.
 11. The driving force transmission device for a four-wheel-drive vehicle according to claim 10, wherein the press member includes a ball cam mechanism in which a plurality of ball cam grooves expanding in a circumferential direction on facing surfaces of a fixed cam plate and a rotation cam plate provided coaxially with the press mechanism are provided and balls are sandwiched between the ball cam grooves, and the ball cam grooves includes: a nonlinear area to move the press member in the shift section; and a linear area to move the press member in the press section, and a displacement in an axial direction to a rotation angle of the rotation cam plate is larger in the nonlinear area than in the linear area.
 12. The driving force transmission device for a four-wheel-drive vehicle according to claim 11, wherein the ball cam mechanism includes a retainer that retains relative positions of the respective balls provided in the circumferential direction.
 13. The driving force transmission device for a four-wheel-drive vehicle according to claim 12, wherein the retainer includes a positioning unit that regulates initial positions of the balls to the ball cam grooves.
 14. The driving force transmission device for a four-wheel-drive vehicle according to claim 10, wherein the press mechanism includes: a rotation cam plate that is provided coaxially with the press member to convert a rotation displacement into an axial direction displacement; a fringe cam that has a cam surface in an circumferential direction and is driven to rotate by the drive source; and a cam follower to be engaged with the fringe cam at an end of an arm unit expanding from the rotation cam plate, the cam surface includes: a nonlinear area to move the press member in the shift section; and a linear area to move the press member in the press section, and a normal direction displacement of the cam follower to a rotation angle of the fringe cam is larger in the nonlinear area than in the linear area. 